World was already the biggest by far when I first started lurking back in July, and it's just getting more dominant. Before, there was quite some diversity in the distribution of generic communities, but nowadays the vast majority of posts that reach the top are from over there.
I really can't see any specific virtue that it has; uptime is not the best (or so I've heard), the moderation is quite lacking (which is demonstrated by the fact that Beehaw defederated them), they make some unpopular moderation choices (like blocking !piracy@lemmy.dbzer0.com), and overall the atmosphere is a lot less... nice than those of smaller instances.
I also feel like it goes against the idea of the Fediverse that one instance has control over most of the platform. Especially on Lemmy, where communities mean that building community within an instance makes so much more sense than elsewhere, and upvotes are federated near perfectly regardless the size of your instance, decentralisation makes a lot of sense. It really just doesn't make sense to me that Lemmy World is where people are going.
Yeah, I guess people have a reflex to always go wherever is the biggest (which doesn't really make sense in the Fediverse).
Mastodon is different, though. Mastodon.social is the default instance and is heavily suggested by the company, while join-lemmy.org lists instances randomly by default. There must be something that inclined users to join it, considering that it gained enough momentum to make up more than half of Lemmy users (not counting alien.top).
Join-lemmy was different at the time. There were only a few instances listed, and most of them where either quite selective in their registration, completely closed, or open. LW was among the last ones.
There was also the trend (and I did it as well) to tell Reddit users to "just go to LW, it's like Reddit" to avoid having to confuse them with federation.